The T430 I bought on ebay came with 320GB hdd, I am going to replace it with a 2.5" 128GB ssd for OS drive. I also see from wiki that this model can take an msata ssd and I can buy ultrabay adapter to add another hdd instead of dvd drive which I'll never user. How much would battery life be lowered if I did this? I am planning to install Linux.

If you set the HD to power off after a few minutes of non-use, it should hardly matter.
My X230 has a 250GB mSATA SSD and a 500GB HDD in the main bay, which powers off after 2 minutes.
Still getting about 6-7 hours out of its 9-cell.

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